This invention relates to a nonglaring mirror or variable reflectance mirror which is functional as an electrochromic device.
A variable reflectance mirror can be produced by utilizing a liquid crystal. In principle, such a mirror is a liquid crystal cell having a transparent electrode film on the front side and a reflecting electrode film on the opposite side, and a mixture of a liquid crystal and a pigment occupies the space between the two electrode films. Reflectance of this mirror can be varied by producing an appropriate electric field between the two electrocdes to change the state of the liquid crystal and also the light absorption of the pigment to thereby vary the light transmittance of the liquid crystal layer. However, liquid crystal mirrors have some disadvantages, which depend on the type of the employed liquid crystal, such as blurring of reflected images in reduced reflectance mode use of the mirror or difficulty in sufficiently lowering the reflectance. In another aspect, it is a weakpoint of the liquid crystal mirror that the color tone of the mirror in reduced reflectance mode is inherent to the pigment and is invariable.
Recently electrochromic nonglaring mirrors are under development. Some electrochromic materials undergo two-stage oxidation and two-stage reduction and assume a characteristic color by first-stage oxidation and another color by second-stage oxidation. A typical example of such electrochromic materials is Prussian blue, which assumes blue color in its first-stage oxidized state and yellowish brown color in its second-stage oxidized state and becomes colorless by full reduction. However, some problems are involved in electrochromic devices using Prussian blue. First, Prussian blue formed by a usual electrodeposition method is in its first-stage oxidized state so that an electrochemical reduction treatment of the electrode coated with Prussian blue is indispensable. Besides, to effectively and stably utilize the second-stage oxidation of Prussian blue it is necessary to pay special attention to the composition of the electrolyte solution used in the electrochromic device.